High Fat Diet (HFD)-induced obesity is a major contributor to diabetes and cardiovascular disease, but the underlying genetic mechanisms are poorly understood. Here, we use Drosophila to test the hypothesis that HFD-induced obesity and associated cardiac complications have early evolutionary origins involving nutrient-sensing signal transduction pathways. We find that HFD-fed flies exhibit increased triglyceride (TG) fat and alterations in insulin/glucose homeostasis, similar to mammalian responses. A HFD also causes cardiac lipid accumulation, reduced cardiac contractility, conduction blocks and severe structural pathologies, reminiscent of diabetic cardiomyopathies. Remarkably, these metabolic and cardiotoxic phenotypes elicited by HFD are blocked by inhibiting insulin-TOR signaling. Remarkably, reducing insulin-TOR activity by TSC1-2, 4EBP, FOXO) or increasing lipase expression in the myocardium suffices to efficiently alleviate cardiac fat accumulation and dysfunction induced by HFD. We conclude that deregulation of insulin-TOR signaling due to a HFD is responsible for mediating the detrimental effects on metabolism and heart function.

SUMMARY Obesity and metabolic syndrome are associated with an increased risk for lipotoxic cardiomyopathy, which is strongly correlated with excessive accumulation of lipids in the heart. Obesity- and type 2 diabetes-related disorders have been linked to altered expression of the transcriptional cofactor PGC-1α, which regulates the expression of genes involved in energy metabolism. Using Drosophila, we identify PGC-1/spargel (PGC-1/srl) as a key antagonist of high-fat diet (HFD)-induced lipotoxic cardiomyopathy. We find that HFD-induced lipid accumulation and cardiac dysfunction are mimicked by reduced PGC-1/srl function and reversed by PGC-1/srl overexpression. Moreover, HFD feeding lowers PGC-1/srl expression by elevating TOR signaling and inhibiting expression of the Drosophila adipocyte triglyceride lipase (ATGL, Brummer), both of which function as upstream modulators of PGC-1/srl. The lipogenic transcription factor SREBP also contributes to HFD-induced cardiac lipotoxicity, likely in parallel with PGC-1/srl. These results suggest a regulatory network of key metabolic genes that modulates lipotoxic heart dysfunction.

The transcription factor dMyc is the sole Drosophila ortholog of the vertebrate c-myc protooncogenes and a central regulator of growth and cell-cycle progression during normal development. We have investigated the molecular basis of dMyc function by analyzing its interaction with the putative transcriptional cofactors Tip48͞Reptin (Rept) and Tip49͞Pontin (Pont). We demonstrate that Rept and Pont have conserved their ability to bind to Myc during evolution. All three proteins are required for tissue growth in vivo, because mitotic clones mutant for either dmyc, pont, or rept suffer from cell competition. Most importantly, pont shows a strong dominant genetic interaction with dmyc that is manifested in the duration of development, rates of survival and size of the adult animal and, in particular, of the eye. The molecular basis for these effects may be found in the repression of certain target genes, such as mfas, by dMyc:Pont complexes. These findings indicate that dMyc:Pont complexes play an essential role in the control of cellular growth and proliferation during normal development.repression ͉ transcription M yc proteins are essential regulators of growth, proliferation, and apoptosis in metazoans (1-3). These proteins act as transcription factors to control the expression of numerous target genes involved in growth, metabolism, and other processes (4-7). Less is known about the molecular mechanism that allows Myc to control the expression of these targets. In recent years, different modes of gene activation by Myc have been proposed, notably recruitment of chromatin remodelers (8), histone acetylases (e.g., ref. 9), or RNA pol II kinases (10), but the physiological relevance of these different factors for Myc-dependent biological functions needs to be demonstrated. We therefore set out to study the mechanisms of Myc-controlled growth and proliferation during normal development by using Drosophila as a model system. Initially, we focused on the interaction of Myc with two specific components of cofactor complexes, Tip48 and Tip49, because of the availability of null mutations in the corresponding genes [called reptin (rept) and pontin (pont) in flies, respectively].Tip48 and Tip49 are closely related proteins that show a high similarity to the bacterial ATP-dependent AAAϩ super family DNA helicase RuvB. Orthologs of Tip48 and Tip49 have been identified in plants, yeast, and animals (e.g., refs. 11 and 12). Different observations strongly suggest that one major function of the Tip proteins resides in the control of transcription. Initially, vertebrate Tip49 was found to be a Tata-binding protein-interacting protein (13-16); later Tip48 and Tip49 were also shown to interact physically with the different transcription factors ␤-catenin (11, 14), c-Myc (12), E2F1 (only Tip49) (17), and ATF2 (only Tip48) (18), raising the possibility that the Tip proteins could bridge basic transcription machinery and sequence-specific activators. Both proteins were also purified as part of several multiprotein complexes involved i...

Obesity has dramatically increased in prevalence, making it essential to understand its accompanying metabolic changes. Modeling diet-induced obesity in Drosophila melanogaster (fruit flies), we elucidated transcriptional and metabolic changes in w1118 flies on a high-fat diet (HFD). Mass spectrometry-based metabolomics revealed altered fatty acid, amino acid, and carbohydrate metabolism with HFD. Microarray analysis uncovered transcriptional changes in nitrogen metabolism, including CG9510, homolog of human argininosuccinate lyase (ASL). CG9510 knockdown in flies phenocopied traits observed with HFD, namely increased triglyceride levels and decreased cold tolerance. Restoration of CG9510 expression ameliorated observed negative consequences of HFD. Metabolomic analysis of CG9510 knockdown flies confirmed functional similarity to ASL, regulating the balance of carbon and nitrogen metabolism. In summary, we found that HFD suppresses CG9510 expression, a gene required for proper triglyceride storage and stress tolerance. These results draw an important link between regulation of amino acid metabolism and the response to diet-induced obesity.